The present invention relates to a thermal fuse used to prevent equipment from being damaged due to overheat, and a battery pack.
Recently, the advance in technical development of secondary batteries is very remarkable. Particularly, as batteries used for equipment such as cellular phones, PHS, and notebook sized personal computers, secondary batteries which are compact and can be used for a long time per charge are developed and put to practical use. Specifically, batteries such as Nixe2x80x94Cd cells, Nixe2x80x94H cells, Li-ion cells, and Li-polymer cells are increasingly developed and put to practical use, enhancing the development of secondary batteries which are smaller in size and have a long service life.
However, with reduction in size of the batteries, heat is generated due to rapid discharge, for example, caused by short-circuiting between positive and negative electrodes. And it gives rise to a fear of damage or explosion of the batteries. In order to prevent such problem, a thermal fuse is used, which blows when heat is generated due to short-circuiting or the like, thereby assuring safety of the secondary batteries. As a thermal fuse like this, a thermal fuse using a fusible metal is commonly employed. Such fusible metal is attached via an insulating layer to a part which may heat in a battery or power source equipment. When a battery or power source equipment is heated, the fusible metal is melted to break the circuit before the temperature reaches a dangerous level. Thus, discharging or charging of the battery is discontinued to prevent overheating of the battery. Further, breakdown of the power source equipment due to heat can be prevented.
FIGS. 22, 23, 24 show a conventional thermal fuse. In FIGS. 22, 23, 24, the conventional thermal fuse used is a thin-type fuse, comprising lead conductors 40, 41, and an insulating plate 42. The conventional thermal fuse has an insulating plate 42 having a width greater than that of the lead conductor 40, 41.
Inter-terminal distance xe2x80x9cdxe2x80x9d between the lead conductor 40 and lead conductor 41 is less than the width of fusible alloy welding position 46. Therefore, the inter-terminal distance d is shorter than the fusing distance when the thermal fuse is normally fused.
FIG. 21 is a perspective view of a conventional battery pack. The battery pack is used as a square battery pack whose thickness is identical with the width of the insulating plate 42.
The conventional battery pack ranges from 6 mm to 5 mm in thickness. However, with the recent reduction in size and thickness of cellular phones, there has been an increasing demand for pack batteries of smaller and thinner type ranging from 2.5 mm to 4 mm in thickness. However, in a conventional thin-type thermal fuse, there has been a problem such that the width of insulating plate 42 cannot be reduced without reducing the width of lead conductors 40, 41. Also, there has been a limit to size reduction with respect to the lengthwise direction of the insulating plate since the withstand voltage distance between terminals is the above inter-terminal distance xe2x80x9cdxe2x80x9d in an air atmosphere.
Also, in case the conventional thermal fuse is merely reduced in size, deterioration of various characteristics such as bonding strength of each member and thermal response will take place as a matter of course.
The present invention provides a thermal fuse and battery pack which ensure excellent reliability, high quality and low cost, with power less consumed by the wiring in the battery body, even after being reduced in size.
A thermal fuse of the present invention comprises:
(a) a fuse main body having a substrate, a fusible metal and a cover;
(b) a pair of terminals protruding from the fuse main body,
the pair of terminals including a first terminal and a second terminal,
one end of the first terminal protruding from one end of the fuse main body, and
one end of the second terminal protruding from the other end of the fuse main body,
wherein the other end of the first terminal includes a first fusible metal connection;
the other end of the second terminal includes a second fusible metal connection;
the fusible metal is disposed between the first terminal and the second terminal;
one end of the fusible metal is connected to the first fusible metal connection, and the other end of the fusible metal is connected to the second fusible metal connection;
the cover is disposed so as to cover the fusible metal, the first fusible metal connection, and the second fusible metal connection; and
length L1 of the fuse main body positioned between the first terminal and the second terminal and thickness L3 of the main body are in a relationship as follows:
2.0 mm less than L1 less than 8.5 mm 
0.4 mm less than L3 less than 2.5 mm. 
A battery pack of the present invention comprises:
(i) a battery;
(ii) a main boy to accommodate the battery;
(iii) a wire led out of the main body and electrically connected to the battery; and
(iv) a thermal fuse disposed between the wire so as to contact with the main body,
wherein the thermal fuse comprises the above mentioned structure.
Preferably, the width of the first fusible metal connection is less than the width of the first terminal, and the width of the second fusible metal connection is less than the width of the second terminal.
Preferably, the fuse main body further comprises an bonding film disposed between the substrate and the cover, and the bonding film has a third through-hole, while the fusible metal, the first fusible metal connection and the second fusible metal connection are located in the third through-hole.
Preferably, each terminal of the first terminal and the second terminal ranges from 3xc3x971010 Pa to 8xc3x971010 Pa in Young""s modulus, and from 4xc3x97108 Pa to 6xc3x97108 Pa in tensile strength.
A method of manufacturing a thermal fuse of the present invention comprises the steps of:
(a) manufacturing at least one substrate of a strip substrate and a plate substrate;
(b) disposing a first terminal and a second terminal opposed to each other on the substrate,
where one end of the first terminal includes a first fusible metal connection, and one end of the second terminal includes a second fusible metal connection,
the first fusible metal connection and the second fusible metal connection are opposed to each other on the substrate, and
the other ends of the first terminal and the second terminal protrude in the respective directions of the substrate;
(c) placing an bonding film on the substrate with the first terminal and the second terminal disposed thereon, then heating under pressures the substrate and the bonding film thus laminated, thereby bonding the bonding film to the substrate by first weld deposit produced by heating,
where the bonding film has a third through-hole, and the first fusible metal connection and the second fusible metal connection are exposed inside the third through-hole;
(d) disposing a fusible metal between the first fusible metal connection and the second fusible metal connection;
(e) disposing a cover film to cover the fusible metal and the bonding film, then heating the cover film and the bonding film positioned around the fusible metal except the zone where the fusible metal is disposed, thereby bonding the bonding film to the cover film by second weld deposit produced by heating; and
(f) forming a fuse main body by cutting off weld deposit zones, so as to include a part of the weld deposit zone between the cover film and the bonding film bonded by the second weld deposit,
where the fuse main body includes a rise portion and the weld deposit zone, and the fusible metal is located in the rise portion.
By the above configuration, a thermal fuse reduced in size and thickness can be obtained. Further, it is possible to obtain a thermal fuse and battery pack which ensure excellent reliability, high quality and low cost, with power less consumed by the wiring in the battery body, even after being reduced in size.